May 8 (Bloomberg) -- Swine flu investigators in Mexico are trying to understand why some young adults died rapidly from influenza infections that usually kill people who are very young or very old.

While most of Mexico’s 45 confirmed deaths occurred in people with other health conditions that made them vulnerable, swine flu also killed a “limited number” of young, previously healthy adults and needs more investigation, said Sylvie Briand, acting director of the World Health Organization’s global influenza program in Geneva, in a conference call with reporters today.

The Spanish flu of 1918 that killed an estimated 50 million people in the world’s deadliest recorded pandemic also hit healthy, young adults with serious consequences, experts have said. Scientists are reviewing the records of the young, fit patients in Mexico to see whether their care or other health conditions might have factored into the deaths, Briand said.

“We’re still trying to understand who are the high risk groups for this disease,” Briand said. “The more we know about this disease, the more we’ll be able to have better control measures.”

Twenty-six, or 58 percent, of the 45 swine flu deaths in Mexico were people ages 20 to 39, Mexican health authorities said today.

Underlying Health Conditions

About 11 percent of all those who died had heart disease, and 24 percent had diabetes or were obese, conditions that might predispose patients to complications, before catching the virus, according to statistics provided today by the government’s Ministry of Health. The ministry didn’t give the ages of the people with the pre-existing health conditions.

Scientists have proposed that some deaths from flu are brought on in part by an extreme immune response, often called a “cytokine storm,” that occurs when the immune systems of young, robust individuals overreact to infections. About 56 percent of people who died from flu in Mexico showed signs of a “hyper immune reaction” according to health ministry’s figures. The ministry didn’t give details such as the ages of patients suffering the reactions.

Canada confirmed today the death of a woman in her 30s, the country’s first death from the virus known as H1N1, Andre Corriveau, Alberta’s chief medical officer of health, told reporters. It is the first swine flu fatality outside the U.S. and Mexico. WHO, the United Nations health agency, said it confirmed swine flu in 2,500 people and 25 countries. Panama later confirmed its first case, according to Health Minister Rosario Turner.

U.S. Toll

The U.S. confirmed 1,639 cases in 43 states, including two deaths, the CDC said. The U.S. illness number doubled from yesterday as laboratories processed backlogged samples from suspected patients, according to the CDC.

“We’re not out of the woods yet,” President Barack Obama said in an administration-sponsored Spanish-language town hall event at the White House. “We may have to prepare for an even worse flu season sometime in the fall,” he said.

The virus affects youth more than seasonal influenza, and younger patients are entering hospitals, the CDC’s acting director, Richard Besser, said yesterday. Few with swine flu are older than 60, and the median age is 14. It’s possible that older people have greater immunity or that younger people spread the disease on spring break vacation trips to Mexico, he said.

No Sustained Spread

WHO hasn’t seen sustained, person-to-person spread of the disease outside North America, so the agency’s pandemic alert will remain at phase 5, the second-highest level, Briand said. The alert won’t be raised unless there’s evidence of community spread in another part of the world, she said.

Mexico, hardest hit by swine flu, confirmed 45 deaths and 1,364 proven cases, according to Health Minister Jose Cordova. Health officials there said stringent measures beginning with the national closing of schools saved more than 8,000 lives.

Deaths from the virus would have ballooned to 8,605 and more than 30,000 people would have been hospitalized without the restrictions, which also closed restaurants and theaters, according to Oscar Mujica, a senior analyst with the Washington- based Pan American Health Organization, an affiliate of WHO.

Limiting the Disease

“An epidemic of influenza, with a propagation similar to what we have observed in Mexico, without measures of control, would have a potential impact considerably greater than what we have observed,” Mujica said during a press conference in Mexico City today. “If the epidemic seems to be stabilizing and the measures adopted are having an impact, it is imperative to keep carrying out the measures of control that have been adopted.”

Five children younger than 14 were among Mexico’s reported H1N1 deaths and just two were people ages 60 or older, according to statistics provided by the Ministry of Health.

The CDC advised U.S. schools to reopen because the virus has already spread widely in the U.S. and is no more serious than seasonal flu.

“One of the things that’s critically important during an outbreak is to look at which measures work and don’t work,” Besser said today in a telephone conference with reporters today from CDC’s office in Atlanta. “Mexico provides another setting to understand what works, what won’t work, and what we should do should this return in the fall.”

Changing Habits

About two-thirds of Americans said they washed their hands or used a hand-sanitizer more frequently to avoid swine flu infection, according to a poll released today by Harvard University’s School of Public Health in Boston. About one in four of those surveyed said they have avoided air travel and public places, said Robert Blendon, the Harvard professor who led the survey.

“This outbreak has permeated a lot of American life,” Blendon said on the CDC conference call.

Both U.S. deaths occurred in Texas, and the victims, a 33- year-old woman and a 22-month-old boy, had underlying health conditions that made them vulnerable, CDC scientists said yesterday.

The spread of the disease and its severity, particularly in healthy people, will play a role in determining the need for vaccine, WHO officials have said. A WHO panel will meet May 14 to decide whether drugmakers should begin producing hundreds of millions of doses of a vaccine against the new illness.

Keiji Fukuda, the WHO’s assistant director-general of health, security and environment, said swine flu may spread to at least one-third of the world’s 6 billion people in the next year.

‘Enormous Numbers’

“Even if the illnesses appear relatively mild at the individual level, the global population level adds up to enormous numbers,” Fukuda said yesterday.

Hong Kong, which confirmed its first case of swine flu on May 1, released about 350 people after a seven-day quarantine. Most were guests and employees of the hotel where the infected person stayed.

Disease trackers are monitoring 88 cases in Spain and 34 in the U.K. to determine whether the virus has established itself outside North America. Such a finding would prompt the WHO to declare a pandemic, the first since 1968, the agency said.

“We are very early in the epidemic,” Marie-Paule Kieny, director of the WHO’s initiative on vaccine research, said yesterday. “We have recommended for all manufacturers to put everything into place to be able to start manufacturing the vaccines.”

Authorities advised hand washing, hygiene and staying home if sick as the most effective ways to control the outbreak. The WHO and CDC said closing borders or killing animals are costly steps that wouldn’t slow the spread of the flu.

To contact the reporters on this story: John Lauerman in Boston at jlauerman@bloomberg.net.

Last Updated: May 8, 2009 18:52 EDT